The present invention relates to a negative electrode material for a non-aqueous electrolyte secondary battery and a method for producing the same. Also, the present invention relates to a negative electrode containing the above negative electrode material, as well as to a non-aqueous electrolyte secondary battery having such negative electrode and exhibiting a high capacity and a long cycle life.
As the negative electrode for a non-aqueous electrolyte secondary battery, metallic lithium or lithium compounds have been intensively studied because they can realize a high energy density and high voltage. On the other hand, as the positive electrode, oxides and chalcogenides of transition metals such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2, MoS2 and the like have been studied. These materials are known to have a layered or tunneled structure that allows free intercalation and deintercalation of lithium ions.
There is a drawback that, when metallic lithium is used in the negative electrode, a deposition of lithium dendrites occurs on the surface of the metallic lithium in the negative electrode during charging, which reduces the charge/discharge efficiency of the battery or causes internal short-circuiting due to contact between formed lithium dendrites and the positive electrode. For this reason, lithium ion batteries have recently been put into practical use that use in the negative electrode graphite type carbon materials which can reversibly absorb and desorb lithium, and have an excellent cycle life and good safety, though the carbon materials have a smaller capacity than metallic lithium.
However, when carbon materials are used in the negative electrode, the fact that the practical capacity thereof is as small as 350 mAh/g and that the theoretical density thereof is as low as 2.2 g/cc presents an obstacle for achieving batteries with a high capacity. Consequently, the use of metallic materials having a higher practical capacity is desired as the negative electrode material.
On the other hand, when metallic materials are used as the negative electrode active material, there is a problem that the active material repeatedly expands and contracts, thereby pulverizing the active material along with absorption and desorption of lithium. The pulverized active material particles lose contact with other particles of the active material or electrically conductive agent in the negative electrode to become inactive, which reduces the electrical conductivity of the negative electrode as well as the capacity.
For solving this problem, there has been proposed a method of allowing a phase absorbing lithium to coexist with a phase not absorbing lithium in one particle of the active material (Japanese Laid-Open Patent Publication No. Hei 11-86854). In this case, the phase not absorbing lithium relaxes the stress caused in the active material particle due to absorption of lithium, and thereby suppresses expansion and pulverization of the active material.
Further, there has been a suggestion in which two or more phases absorbing lithium are allowed to coexist in one particle of the active material, intending to relax the stress by the change in the structure during absorbing lithium of each phase (Japanese Laid-Open Patent Publication No. Hei 11-86853). In this case, it is considered that, since a plurality of minute phases exist in the active material particles, it is possible to let the stress go to the interface of the crystal grains at the time of absorption of lithium.
However, negative electrode active materials capable of sufficiently suppressing the expansion and pulverization of the active material by relaxing the stress caused by absorption of lithium, have not been achieved so far because an appropriate synthesizing method of the active material has not been found.
For example, by conventional methods such as atomizing method and roll quenching method, the crystal grain size of each phase is as large as several microns at minimum, and therefore effective stress relaxation is not possible.
Also, by conventional methods of mechanically applying a shearing force to raw materials comprising simple substances such as mechanical alloying method and mechanical grinding method under vacuum or argon atmosphere, crystal grains in the range of less than several microns to several nanometers can be formed; however, these methods are not practical since a variety of phases of substances having a variety of compositions are formed and the control of phase formation is difficult.
An object of the present invention is to provide a negative electrode material that sufficiently suppresses the expansion or pulverization thereof by relaxation of the stress caused during absorption of lithium in the negative electrode material. Also, the present invention has another object of providing a non-aqueous electrolyte secondary battery which is equipped with a negative electrode containing the above negative electrode material and which has a high capacity, a long cycle life and excellent high rate discharge characteristics.
The present invention relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery including a step of, applying a shearing force or a shearing stress to a raw material comprising an intermetallic compound under a nitrogen-containing atmosphere to make the intermetallic compound react with nitrogen. The raw material comprising an intermetallic compound is well known in the art.
The intermetallic compound comprises: at least one element(A) which reacts with nitrogen and forms a nitride, but is substantially non-reactive with lithium; and at least one element(B) that is substantially non-reactive with nitrogen, but reacts with lithium.
The intermetallic compound forms a mixture containing at least one nitride of element(A), and at least one substance of element(B) by the above reaction with nitrogen. It is preferable that 30% or more of element(A) in the intermetallic compound converts into a nitride of element(A) by the step of applying a shearing force to the raw material.
The present invention also relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery including the steps of: providing an intermetallic compound comprising at least one element(A) selected from the group A consisting of Ce, Co, Cr, Fe, La, Mn, Mo, Nb, P, Sc, Sr, Ta, Ti, V, Y, Yb, Zr, B, Ca, Mg, Na and Zn, and at least one element(B) selected from the group B consisting of Ge, Sn, Pb, Sb and Bi, and applying a shearing force to the intermetallic compound under a nitrogen-containing atmosphere to make the intermetallic compound react with nitrogen, thereby forming a mixture containing at least one nitride of element(A) and at least one substance of element(B).
The aforementioned nitrogen-containing atmosphere is preferably an atmosphere of a gas containing 50% by volume or more of nitrogen.
The pressure of the aforementioned gas is preferably 1.0xc3x97105 Pa or more.
The step of applying a shearing force to the raw material, as described above, is preferably performed by a mechanochemical method using for example a ball milling system. The mechanochemical method is obvious to one of ordinary skill in the art.
The present invention also relates to a method for producing a negative electrode material for a non-aqueous electrolyte secondary battery including the steps of: providing an intermetallic compound comprising at least one element(A) selected from the group A consisting of Ce, Co, Cr, Fe, La, Mn, Mo, Nb, P, Sc, Sr, Ta, Ti, V, Y, Yb, Zr, B, Ca, Mg, Na and Zn, and at least one element(B) selected from the group B consisting of Ge, Sn, Pb, Sb and Bi, mixing the intermetallic compound with a compound containing nitrogen to obtain a raw material mixture comprising the intermetallic compound and the compound containing nitrogen, and applying a shearing force to the resultant raw material mixture to make the intermetallic compound react with the compound containing nitrogen, thereby forming a mixture containing at least one nitride of element(A), and at least one substance of element(B).
The aforementioned step of applying a shearing force to the raw material mixture is preferably performed by a mechanochemical method and is preferably conducted under an inert atmosphere containing argon.
The compound containing nitrogen preferably contains at least one element(C) selected from the group C consisting of B, Ba, C, Ca, Ce, Co, Cr, Fe, La, Li, Mg, Mn, Na, Nb, Sr, Ta, V, Y and Yb.
Alternatively, the compound containing nitrogen preferably contains at least one element(D) selected from the group D consisting of Al, Ga and Si. In this case, since an active simple substance of element(D) is formed during the nitrogenation reaction of the raw material, a negative electrode material having a high capacity can be obtained.
The average particle size of the above-mentioned intermetallic compound is preferably 200 xcexcm or less.
The above-mentioned intermetallic compound is preferably represented by the general formula: A1xB1, where A1 is at least one element(A) selected from the group A, B1 is at least one element(B) selected from the group B, and 0.25xe2x89xa6xxe2x89xa64.
In the general formula: A1xB1, A1 is preferably Ti or Zr, and B1 is preferably Sn.
The present invention also relates to a negative electrode material for a non-aqueous electrolyte secondary battery comprising: at least one nitride of element(A) selected from the group A consisting of Ce, Co, Cr, Fe, La, Mn, Mo, Nb, P, Sc, Sr, Ta, Ti, V, Y, Yb, Zr, B, Ca, Mg, Na and Zn; and at least one substance of element(B) selected from the group B consisting of Ge, Sn, Pb, Sb and Bi; wherein the average crystal grain size of the above nitride is 0.001 to 0.1 xcexcm.
In this negative electrode material, the content of nitrogen is preferably 1.0% by weight or more.
The present invention also relates to a negative electrode for a non-aqueous electrolyte secondary battery comprising the aforementioned negative electrode material. The content of the negative electrode material in the negative electrode is generally 15 to 95% by weight. The negative electrode further comprises a binder, an electrically conductive agent and the like.
The present invention further relates to a non-aqueous electrolyte secondary battery comprising the aforementioned negative electrode, a positive electrode capable of absorbing and desorbing lithium, and a non-aqueous electrolyte.